Dont believe everything you read. Yet another example of rules that are not enforced and nobody care about.

All my work bookings through AMEX end up on BA with my first and second names concatenated without a space or capital letter on the second name. Firstnamesecondname Surname.

I use the abbreviated version (Tim) of my full first name although my passport of course has the full version. The abbreviated version is what is entered on the BA EC therefore this is what BA knows me as and the BPs always have this printed on. On BA it is never an issue (except for one rather pedantic check-in agent at GOA) but never, ever anywhere else on BA's network. Even using SSCI, BA's systems are OK with my passport having the full version but the booking being the abbreviated one

But on pretty much all other airlines it is always rejected, particularly on Flybe and Swiss whereby it is insisted the full version is used on any booking otherwise the computer says "no"

I suspect that in practice, this language is more intended to prompt Ms Alexandra Foster to not book as "Sasha Foster" even though that's what she's always been called by her family and friends.

My middle name is on my passport but I have never used it since I left school 45 years ago and it has never been an issue in the US or anywhere else in the world.

yep. Or I get the NMN in that field which sometimes leads to other questions.

I have a middle name in my passport but have only ever entered it in the ESTA and similar forms, never for a flight booking. No problem so far.
Funny sidenote: I was issued a credit card once that had not only my name above the cc# but also a good part of my address.

My global entry only has my first and last names in spite of my middle name being on my passport. In fact, because of this I've had to take the middle name out of my reservations and pax profile with AA in order to be recognized for TSA Precheck. So, not a problem in the US

I have no middle name neither does anyone in my family (my grandad used to say "you have 2 names. Isn't that enough?"). I once worked for a US company where their HR system couldn't cope with someone without a middle name - so I was issued with one!

I have the dubious honour of having five surnames, one of which is three words long (de Las Mellizas) and two of which are hyphenated. All in my full name is 61 characters long. Plays merry hell with every system let alone airlines. Fortunately, I have an observation added to my passport that I only regularly use two surnames, but those are the hyphenated ones so it still doesn’t work well with many systems.

That said, I’ve never had an issue with my middle name not being on a ticket nor my (listed) surnames being concatenated without the hyphen.

I work for a company large enough to have its own travel agency. For some reason when I was employed and got an account with them, my middle name didn’t get a capital letter. Other than that it’s perfectly correct.

I have had problems with this on AA on multiple occasions. I find this bizarre as a passport is all caps.

If you read the novels by Lee Childs, the hero always gives his name to the authorities as Jack no middle name Reacher, so I guess a 3rd name (at least) is usually expected.

Respected fellow members
I think this middle name issue and clarification is clear as mud .
I do believe dometic travel ( in USA) no one bothered much about middle name in the ID or Boarding pass.
When it comes to international travel, as i understand your passport is your ID.

Having said that , my friend is in tricky situation
In his current passport from country A, is with first name , mddile (full) name and last name ( Surname/family name )
In all other documents in USA Driver License etc First Name , middle initial and last name
Now he has applied for citizenship with First name , middle initial and last name, eventually he will be receiving ( may be within 3-6 months) the passport with middle initial only.
if he want to book tickets for future ( in 3-6 months time ) international travel , what he should do ?
As per current passport - First Name + Middle (full) Name + Last Name ? What if before his travel date, he obtains the citizenship and new passport with just middle initial ?
Other way around - if he books the ticket with just middle initial - and if could not new passport before his travel date , is there will be an issue ?

Any insight is much appreciated.
Thanks

It really shouldn't be an issue either way you put it.

If your friend is travelling on BA, all BA really care about is that the first name and family name on the reservation match the passport they are presenting to travel. As someone with a second given (middle) name in my passport, I never use it when travelling or making flight reservations and nobody has ever flagged me for it. I'm more concerned that my BA Exec Club details won't match if I do add my middle name as I can't remember how I set up my Exec Club membership if truth be told. If flying to or from the USA then the Department of Homeland Security will already have vetted the ID of all passengers well before check-in opens. Anyone they want to flag will be flagged and it won't be because you chose not to add your middle name to your flight reservation when you have one in your passport - I can contest to that as someone who's travelled to/from the US for as long as I care to remember without ever using my middle name in my flight reservations. On any airline.